Dryer venting issues

[r1100rider]

About as off topic as it can get, eh?

I own a 2 story (separated) house. I rent out the lower apartment and live in the upper. Both have washer/dryer hook ups and both are used. The contractor who renovated the house when I moved in ran both dryer vents to a single Y connector and vented them outside. There is no outside access to the vent as it's under a porch. About 6 months ago, my tenant hooked up her dryer and I noticed, every time she'd turn it on, it would partially vent through MY dryer. This is obviously unacceptable. I had a handyman in today to run an outside vent to the rear of the house.

Here's where it gets tricky. The lower 1/2 of the house is poured concrete finished in stone. The upper half is typical construction (all wood). I was planning on running the outside vent downstairs as it has a dropped ceiling thus easy access. Turns out nobody has anything that will drill a 4" hole through solid concrete and the only way to get to the little bit of wood above it (without going through my floor) is to cut a 4" hole through a floor joist. There's a pocket behind the joist that would allow the outside vent but I don't want anybody cutting 4" out of a 6" joist. Just doesn't seem like a good idea. My dryer is on an INSIDE wall so I can't vent it as most would.

I wish I had taken pics but hopefully you get the idea. Does anybody have any suggestions? I can probably hire a carpenter to go through the joist and shore it up however it needs to be done but I can see some serious $$$ going that route. I can have another vent run through the front of the house but I don't even know how the original contractor did that as it's probably 4' from the wall to whatever he drilled through. In hindsight, that probably isn't even the best idea as I don't see any way to secure/seal the pipe.

I'm hoping one of you knowledgable folks has some suggestions.

HAGO!

they make a heat pump dryer now that condensates the moisture and you don't have to vent

 

[IdahoMtnSpyder]

Instead of going through the joist can you go parallel to the joist to the rim board and out?

 

[MONK]

Instead of going through the joist can you go parallel to the joist to the rim board and out?

Unfortunately, no. I'd end up with an add'l 50' run and I'd still probably end up in concrete.